In order to facilitate fitting of the ball head in its seating, it has previously been proposed to form the latter in two removable parts. That is for example so in the arrangements disclosed in the specifications of French published patent applications Nos. FR 1 595 740A and FR 2 606 341A. In accordance with the first mentioned of these two specifications, the ball head seating comprises two half shell elements which are separate and which are arranged to receive and then, after being fitted together, to hold a ball head in position, the whole being locked by means of an external gripping ring which surrounds the two half shell elements.
The arrangement described in FR 2 606 341A is of substantially the same type, except that the two half shells form an external screw with which a nut is arranged to cooperate, so as to hold it in position and also for unlocking purposes.